question on a 90 amp vs 120 amp alternator

You'd have to find the specs on the specific alternators you are interested in. Just being bigger is no guarantee that it's better at low RPM. However I'd have to say that "most" newer designs like ND are likely better than most any of the 60's-70's stuff

Frankly I'm surprised that a ND 90 does not satisfy you. You might want to do some troubleshooting first, like:

What size are the pulleys and determine if it's spinning fast enough

Put it under heavy load, like, idle, with all accessories, heater, headlights, etc running. Measure voltage between alternator output stud and the battery, and see what the voltage drop is on the charge wire?